Substitution mutations are point mutations that substitute an existing nucleotide with a different nucleotide.
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A substitution or point mutation is the most limited type of mutation, whereby a single base [[nucleotide]] is replaced with another nucleotide. Sometimes the expression "point mutation" can also include the addition or subtraction of a single base pair, which is also referred to as an "indel".
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The sickle-cell disease is caused by a single point mutation in one allele of the beta [[hemoglobin]] gene, whereby a CCT codon is converted into TCT. The TCT then mistakenly encodes the [[amino acid]] serine instead of proline.